1. Field of the Invention
The present invention relates to a pattern testing apparatus which tests a digitized video signal derived from an image of a two-dimensional pattern such as a mask or a reticle used in the IC manufacture, with respect to a design pattern to detect a defect.
2. Description of the Prior Art
Prior art pattern testing apparatus which converts a two-dimensional pattern such as a mask or a reticle used in the IC manufacture to an analog video signal by an imaging device and tests the two-dimensional pattern based on a digitized video signal derived from the analog video signal uses one of the following methods:
(1) Two digitized video signals derived by imaging two adjacent chips are compared. PA1 (2) A digitized video signal derived by imaging a pattern under test is compared with a design value. PA1 (3) If a pattern under test has a characteristic which is not included in characteristics of a design pattern (pattern width, pattern angle, etc.), a defect is detected.
In the method (1) above, if the adjacent chips have a common defect, such a defect cannot be detected. Accordingly, a perfect detection of defect cannot be attained unless a normal chip is first selected and other chips are compared with the selected normal chip.
In the method (2) above, a huge volume of data must be stored in order to compare the pattern under test with the design value for each picture cell. In addition, due to misregistration between the design value and the pattern under test, only a defect which is larger than a certain area can be detected.
The method (3) above is relevant to the present invention. It utilizes a feature that a pattern width and a pattern-to-pattern interval of a pattern printed on the mask or reticle are always wider than a design width or a width of a pattern rule and an angle between patterns is constant. If the pattern under test has a narrower pattern than the width of the pattern rule or an angle which is not found in the pattern rule, a defect is detected. Since this method relies only on the characteristics of the pattern, no registration is required and a discrete defect which is smaller than the width of the pattern rule and a defect having a smaller pattern than the width of the pattern rule can be detected with a high precision by a simple logic circuit. However, even in the method (3) above, a special portion of a normal pattern such as an end of a right angle edge having a slope of 45 degrees may be misjudged as a defect.